1. Field of the Invention
This invention relates generally to a brazing structure of an aluminum-made heat exchanger formed by integrally brazing alternately laminating first and second metallic plates that are mainly made of aluminum, and particularly to a brazing structure of a water cooling type oil cooler capable of preventing corrosion of a passage wall on which cooling water flows as medium for cooling oil.
2. Description of the Related Art
Conventionally, there is an aluminum-made heat exchanger such as a water cooling type oil cooler for cooling engine oil (hereinafter, referred to as oil) by exchanging heat with engine cooling water. As shown in FIG. 7, such a water cooling type oil cooler is formed by integrally brazing laminated several pairs of first and second plates 101 and 102 that are formed from aluminum alloy by plastic working (pressing). For instance, one first plate 101 forms a cooling water passage 103 with a second plate 102 adjacent thereto at a lower side thereof, and forms an oil passage 104 with another second plate 102 adjacent thereto at an upper side thereof.
In this water cooling type oil cooler, cooling water deteriorates to have high corrosiveness and to easily cause pitting corrosion on the first and second plates defining the cooling water passage as passage walls, and accordingly, a lifetime of a product is shortened. To prevent the pitting corrosion of the passage walls, as shown in FIG. 7, the passage walls are clad with sacrifice corrosion layers 111, 121. Incidentally, reference numerals 105, 106 respectively indicate oil passages formed in the first and second plates by pressing. However, in the conventional water cooling type oil cooler, as shown in FIG. 7, because no brazing filler metal exists at the joint portion between the first and second plates 101, 102, the first and second plates 101, 102 cannot be bonded together by brazing.
As a countermeasure against the above problem, as shown in FIG. 8A, it is conceivable to adopt a plate composed of a base member (core member) 110 made of aluminum alloy, one surface of which is clad with sacrifice corrosion material 113 and further both surfaces of which are clad with brazing filler metal layers 114, 115. This plate is, however, accompanied by high material cost since it is composed of four layers.
As shown in FIG. 8B, it can also be conceivable to adopt a three-layered plate including the core member (core member) 110 made of aluminum alloy, both surfaces of which are clad with brazing filler metal layers 116, 117 containing Zn that can serve as a sacrifice corrosion material simultaneously. However, when this plate is integrally brazed within a heating furnace such as a vacuum furnace, Zn component is liable to evaporate and flows into a joint part of the plate. Because of this, it is difficult to control the amount of the sacrifice corrosion material covering the passage walls of the cooling water passage. This may result in insufficient corrosion resistance.
As a result, pitting corrosion may be produced on the passage walls of the cooling water passage, or the brazing filler metal layers may be corroded first to cause insufficient sealing that allows communication between the oil passage and the cooling water passage. In consequence, oil can be mixed with cooling water.
It can also be conceivable to adopt first and second plates each having a three-layered structure and joined together as shown in FIG. 9. The first and second plates are respectively composed of first and second plates 101, 102, one surface of which is clad with sacrifice corrosion material 111, 112, and the other surface of which is clad with brazing filler metal 120, 122. The three-layered first and second plates are formed by pressing (burring) processing to form, for instance, burring portions 119, 129 defining oil communication passages 105, 106 through which two oil passages 104 communicate each other.
In this oil cooler, the outer wall surface of the burring portion 129 of the second plate is brazed to the inner wall surface of the burring portion 119 of the first plate. However, it is difficult to raise accuracy of a clearance between the inner wall surface of the burring portion 119 and the outer wall surface of the burring portion 129. For instance, when the burring portion 129 is inserted into the burring portion 119 for assembling, the burring portion 119 may expand outward or bent inward to produce brazing failures at many points. The brazing failures lessen the sealing property at the joining portion between the first and second plates, and the joining strength of the oil cooler.